Method for direct modulation of the spinothalamic tract

ABSTRACT

A method for treating a patient suffering from chronic neuropathic pain, comprises epidurally applying electrical stimulation energy to a spinothalamic tract of the patient, thereby treating the chronic neuropathic pain. The method may further comprise increasing the activation threshold of a side-effect exhibiting neural structure relative to the activation threshold of the spinothalamic tract of the patient, wherein the electrical stimulation energy is applied to the spinothalamic tract of the patient while the activation threshold of the neural structure relative to the spinothalamic tract is increased, thereby treating the chronic neuropathic pain without stimulating the neural structure.

RELATED APPLICATION DATA

The present application claims the benefit under 35 U.S.C. §119 to U.S. provisional patent application Ser. No. 61/569,214, filed Dec. 9, 2011. The foregoing application is hereby incorporated by reference into the present application in its entirety.

FIELD OF THE INVENTION

The present invention relates to tissue stimulation systems, and more particularly, to spinal cord stimulation systems.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. For example, Spinal Cord Stimulation (SCS) techniques, which directly stimulate the spinal cord tissue of the patient, have long been accepted as a therapeutic modality for the treatment of chronic neuropathic pain syndromes, and the application of spinal cord stimulation has expanded to include additional applications, such as angina pectoralis, peripheral vascular disease, and incontinence, among others. Spinal cord stimulation is also a promising option for patients suffering from motor disorders, such as Parkinson's Disease, Dystonia and essential tremor.

An implantable SCS system typically includes one or more electrode-carrying stimulation leads, which are implanted at a stimulation site in proximity to the spinal cord tissue of the patient, and a neurostimulator implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. The neurostimulation system may further include a handheld patient programmer to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The handheld programmer may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.

Thus, programmed electrical pulses can be delivered from the neurostimulator to the stimulation lead(s) to stimulate or activate a volume of the spinal cord tissue. In particular, electrical stimulation energy conveyed to the electrodes creates an electrical field, which, when strong enough, depolarizes (or “stimulates”) the neural fibers within the spinal cord beyond a threshold level, thereby inducing the firing of action potentials (APs) that propagate along the neural fibers to provide the desired efficacious therapy to the patient.

As discussed, SCS may be utilized to treat patients suffering from chronic neuropathic pain. To this end, electrical stimulation is generally applied to the dorsal column (DC) nerve fibers, which is believed to inhibit the perception of pain signals via the gate control theory of pain by creating interneuronal activity within the dorsal horn that inhibits pain signals traveling from the dorsal root (DR) neural fibers that innervate the pain region of the patient up through the spinothalamic tract of the spinal cord to the brain. Consequently, stimulation leads are typically implanted within the dorsal epidural space to provide stimulation to the DC nerve fibers. Such techniques of treating pain may be beneficial for some patients initially, but it has decreasing efficiency over time. This may be because the natural neural interconnections may be minimally present in some patients, or because the natural neural interconnections may be rendered ineffective by the pathological condition.

Some prior art techniques, such as those disclosed in U.S. Patent Publication Nos. 2005/0055065 and 2004/0162594, propose directly stimulating the spinothalamic tract to inhibit the pain signals from traveling up the spinothalamic tract to the brain. However, the spinothalamic tract in these cases are subdurally stimulated, which requires a more difficult, complex, and risky surgical procedure.

Although it would be desirable to epidurally stimulate the spinothalamic tract, various sensitive neural structures, including the dorsal root (DR) nerve fibers and ventral root (VR) nerve fibers, are in close proximity to or even overlap the spinothalamic tract. As a result, epidural stimulation of the spinothalamic tract to treat chronic neuropathic pain may be difficult to accomplish without also inadvertently creating side-effects, e.g., in the form of uncomfortable muscle contractions and pain resulting from the inadvertent stimulation of the DR and/or VR nerve fibers.

SUMMARY OF THE INVENTION

In accordance with the present inventions, a method for treating a patient suffering from chronic neuropathic pain is provided. The method comprises epidurally applying electrical stimulation energy to a spinothalamic tract of the patient, thereby treating the chronic neuropathic pain. Preferably, the electrical stimulation energy is applied to a region of the spinothalamic tract of the patient rostral to a spinal nerve that innervates a painful region of the patient. In one method, the electrical stimulation energy is applied to the spinothalamic tract by at least one electrode implanted within the ventral-lateral quadrant of an epidural space of the patient.

The method optionally comprises increasing the activation threshold of a side-effect exhibiting neural structure (e.g., ventral root (VR) nerve fibers and/or dorsal root (DR) nerve fibers) relative to the activation threshold of the spinothalamic tract of the patient, in which case, the electrical stimulation energy is applied to the spinothalamic tract of the patient while the activation threshold of the neural structure relative to the spinothalamic tract is increased, thereby treating the chronic neuropathic pain without stimulating the neural structure. The activation threshold of the neural structure relative to the activation threshold of the spinothalamic tract may be increased by applying electrical suppression energy to the neural structure.

In one example, the electrical stimulation energy is applied to spinothalamic tract by sinking electrical current at a first electrode adjacent the spinothalamic tract, thereby treating the chronic neuropathic pain, and the electrical suppression energy is applied to the neural structure by sourcing at least a portion of the electrical current from a second electrode adjacent the neural structure, thereby increasing the activation threshold of the neural structure. The method may optionally comprise sinking at least a portion of the electrical current sourced at the second electrode adjacent the neural structure into a remote electrode.

The electrical suppression energy may be applied to another neural structure adjacent the spinothalamic tract to increase the activation threshold of the other neural structure by sourcing at least another portion of the electrical current from a third electrode adjacent the other neural structure, in which case, the electrical stimulation energy may be applied to the spinothalamic tract of the patient while the activation threshold of the other neural structure is increased, thereby treating the chronic neuropathic pain without stimulating the other neural structure. In this case, the neural structure may be a DR nerve fiber and the other neural structure may be a VR nerve fiber, or the neural structure may be a first VR nerve fiber and the other neural structure may be a second VR nerve fiber. The first electrode may be a center electrode, and the second and third electrodes may be flanking electrodes of a tripole configuration located between the neural structures.

In another example, the activation threshold of the neural structure may be increased by applying a sub-threshold, hyperpolarizing conditioning pre-pulse to the neural structure, and electrical stimulation energy may be applied to the spinothalamic tract by applying a depolarizing stimulation pulse to the spinothalamic tract. The duration of the conditioning pre-pulse may be relatively short (e.g., less than 200 μs).

In still another example, the activation threshold of the neural structure is increased by applying electrical background energy to the neural structure in accordance with at least one stochastic parameter to the neural structure. If the electrical background energy comprises pulses, the stochastic parameter(s) may comprise at least one of an interpulse interval, pulse amplitude, pulse shape, and pulse duration. If the electrical background energy comprises white noise, the stochastic parameter may be frequency.

In yet another example, the activation threshold of the neural structure is increased by applying time-varying electrical energy having a frequency and amplitude that increases the activation threshold of the neural structure. In one method, the electrical stimulation energy epidurally applied to the spinothalamic tract is the time-varying electrical energy, and the time-varying electrical energy has a frequency and amplitude that increases the activation threshold of the neural structure, while stimulating the relatively spinothalamic tract.

Other and further aspects and features of the invention will be evident from reading the following detailed description of the preferred embodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodiments of the present invention, in which similar elements are referred to by common reference numerals. In order to better appreciate how the above-recited and other advantages and objects of the present invention are obtained, a more particular description of the present invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 is a plan view of a Spinal Cord Stimulation (SCS) system constructed in accordance with one embodiment of the present inventions;

FIG. 2 is a plan view of the SCS system of FIG. 1 in use within a patient;

FIG. 3 is a plan view of an implantable pulse generator (IPG) and three percutaneous stimulation leads used in the SCS system of FIG. 1;

FIG. 4 is a plan view of an implantable pulse generator (IPG) and a surgical paddle lead used in the SCS system of FIG. 1;

FIG. 5 is a peripheral view of the spinal cord and spinal nerves;

FIG. 6 is a cross-sectional view of the spinal cord;

FIG. 7 is a perspective view showing one electrode arrangement relative to the spinal cord and spinal nerves in accordance with the present invention; and

FIG. 8 is a perspective view showing another electrode arrangement relative to the spinal cord and spinal nerves in accordance with the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Turning first to FIG. 1, an exemplary SCS system 10 generally comprises a plurality of stimulation leads 12 (in this case, three), an implantable pulse generator (IPG) 14 (or alternatively RF receiver-stimulator), an external remote control RC 16, a Clinician's Programmer (CP) 18, an External Trial Stimulator (ETS) 20, and an external charger 22.

The IPG 14 is physically connected via one or more lead extensions 24 to the stimulation leads 12, which carry a plurality of electrodes 26 arranged in an array. The stimulation leads 12 are illustrated as percutaneous leads in FIG. 1, although as will be described in further detail below, a surgical paddle lead can be used in place of the percutaneous leads. As will also be described in further detail below, the IPG 14 includes pulse generation circuitry that delivers electrical stimulation energy in the form of a pulsed electrical waveform (i.e., a temporal series of electrical pulses) to the electrode array 26 in accordance with a set of stimulation parameters.

The ETS 20 may also be physically connected via the percutaneous lead extensions 28 and external cable 30 to the neurostimulation leads 12. The ETS 20, which has similar pulse generation circuitry as the IPG 14, also delivers electrical stimulation energy in the form of a pulse electrical waveform to the electrode array 26 accordance with a set of stimulation parameters. The major difference between the ETS 20 and the IPG 14 is that the ETS 20 is a non-implantable device that is used on a trial basis after the neurostimulation leads 12 have been implanted and prior to implantation of the IPG 14, to test the responsiveness of the stimulation that is to be provided. Thus, any functions described herein with respect to the IPG 14 can likewise be performed with respect to the ETS 20.

The RC 16 may be used to telemetrically control the ETS 20 via a bi-directional RF communications link 32. Once the IPG 14 and neurostimulation leads 12 are implanted, the RC 16 may be used to telemetrically control the IPG 14 via a bi-directional RF communications link 34. Such control allows the IPG 14 to be turned on or off and to be programmed with different stimulation parameter sets. The IPG 14 may also be operated to modify the programmed stimulation parameters to actively control the characteristics of the electrical stimulation energy output by the IPG 14. As will be described in further detail below, the CP 18 provides clinician detailed stimulation parameters for programming the IPG 14 and ETS 20 in the operating room and in follow-up sessions.

The CP 18 may perform this function by indirectly communicating with the IPG 14 or ETS 20, through the RC 16, via an IR communications link 36. Alternatively, the CP 18 may directly communicate with the IPG 14 or ETS 20 via an RF communications link (not shown). The clinician detailed stimulation parameters provided by the CP 18 are also used to program the RC 16, so that the stimulation parameters can be subsequently modified by operation of the RC 16 in a stand-alone mode (i.e., without the assistance of the CP 18).

For purposes of brevity, the details of the RC 16, CP 18, ETS 20, and external charger 22 will not be described herein. Details of exemplary embodiments of these devices are disclosed in U.S. Pat. No. 6,895,280, which is expressly incorporated herein by reference.

As shown in FIG. 2, the stimulation leads 12 are implanted within the spinal column 42 of a patient 40. The preferred placement of the electrode leads 12 is adjacent, i.e., resting upon, the spinal cord area to be stimulated. Due to the lack of space near the location where the electrode leads 12 exit the spinal column 42, the IPG 14 is generally implanted in a surgically-made pocket either in the abdomen or above the buttocks. The IPG 14 may, of course, also be implanted in other locations of the patient's body. The lead extensions 24 facilitate locating the IPG 14 away from the exit point of the electrode leads 12. As there shown, the CP 18 communicates with the IPG 14 via the RC 16.

Referring now to FIG. 3, the external features of the stimulation leads 12 and the IPG 14 will be briefly described. Each of the stimulation leads 12 has eight electrodes 26 (respectively labeled E1-E8, E9-E16, and E17-E24). The actual number and shape of leads and electrodes will, of course, vary according to the intended application. Further details describing the construction and method of manufacturing percutaneous stimulation leads are disclosed in U.S. patent application Ser. No. 11/689,918, entitled “Lead Assembly and Method of Making Same,” and U.S. patent application Ser. No. 11/565,547, entitled “Cylindrical Multi-Contact Electrode Lead for Neural Stimulation and Method of Making Same,” the disclosures of which are expressly incorporated herein by reference.

Alternatively, as illustrated in FIG. 4, the stimulation lead 12 takes the form of a surgical paddle lead on which electrodes 26 are arranged in a two-dimensional array in three columns (respectively labeled E1-E5, E6-E10, and E11-E15) along the axis of the stimulation lead 12. In the illustrated embodiment, five rows of electrodes 26 are provided, although any number of rows of electrodes can be used. Each row of the electrodes 26 is arranged in a line transversely to the axis of the lead 12. The actual number of leads and electrodes will, of course, vary according to the intended application. Further details regarding the construction and method of manufacture of surgical paddle leads are disclosed in U.S. patent application Ser. No. 11/319,291, entitled “Stimulator Leads and Methods for Lead Fabrication,” the disclosure of which is expressly incorporated herein by reference.

In each of the embodiments illustrated in FIGS. 3 and 4, the IPG 14 comprises an outer case 40 for housing the electronic and other components (described in further detail below). The outer case 40 is composed of an electrically conductive, biocompatible material, such as titanium, and forms a hermetically sealed compartment wherein the internal electronics are protected from the body tissue and fluids. In some cases, the outer case 40 may serve as an electrode. The IPG 14 further comprises a connector 42 to which the proximal ends of the stimulation leads 12 mate in a manner that electrically couples the electrodes 26 to the internal electronics (described in further detail below) within the outer case 40. To this end, the connector 42 includes one or more ports (three ports 44 or three percutaneous leads or one port for the surgical paddle lead) for receiving the proximal end(s) of the stimulation lead(s) 12. In the case where the lead extensions 24 are used, the port(s) 44 may instead receive the proximal ends of such lead extensions 24.

The IPG 14 includes pulse generation circuitry that provides electrical conditioning and stimulation energy in the form of a pulsed electrical waveform to the electrode array 26 in accordance with a set of stimulation parameters programmed into the IPG 14. Such stimulation parameters may comprise electrode combinations, which define the electrodes that are activated as anodes (positive), cathodes (negative), and turned off (zero), percentage of stimulation energy assigned to each electrode (fractionalized electrode configurations), and electrical pulse parameters, which define the pulse amplitude (measured in milliamps or volts depending on whether the IPG 14 supplies constant current or constant voltage to the electrode array 26), pulse width (measured in microseconds), pulse rate (measured in pulses per second), and burst rate (measured as the stimulation on duration X and stimulation off duration Y).

Electrical stimulation will occur between two (or more) activated electrodes, one of which may be the IPG case 40. Simulation energy may be transmitted to the tissue in a monopolar or multipolar (e.g., bipolar, tripolar, etc.) fashion. Monopolar stimulation occurs when a selected one of the lead electrodes 26 is activated along with the case 40 of the IPG 14, so that stimulation energy is transmitted between the selected electrode 26 and the case 40. Bipolar stimulation occurs when two of the lead electrodes 26 are activated as anode and cathode, so that stimulation energy is transmitted between the selected electrodes 26. For example, an electrode on one lead 12 may be activated as an anode at the same time that an electrode on the same lead or another lead 12 is activated as a cathode. Tripolar stimulation occurs when three of 15 the lead electrodes 26 are activated, two as anodes and the remaining one as a cathode, or two as cathodes and the remaining one as an anode. For example, two electrodes on one lead 12 may be activated as anodes at the same time that an electrode on another lead 12 is activated as a cathode.

The stimulation energy may be delivered between electrodes as monophasic electrical energy or multiphasic electrical energy. Monophasic electrical energy includes a series of pulses that are either all positive (anodic) or all negative (cathodic). Multiphasic electrical energy includes a series of pulses that alternate between positive and negative. For example, multiphasic electrical energy may include a series of biphasic pulses, with each biphasic pulse including a cathodic (negative) stimulation pulse and an anodic (positive) recharge pulse that is generated after the stimulation pulse to prevent direct current charge transfer through the tissue, thereby avoiding electrode degradation and cell trauma. That is, charge is conveyed through the electrode-tissue interface via current at an electrode during a stimulation period (the length of the stimulation pulse), and then pulled back off the electrode-tissue interface via an oppositely polarized current at the same electrode during a recharge period (the length of the recharge pulse).

Referring now to FIGS. 5 and 6, the portions of the spinal cord 100 that are relevant to the present inventions will be described. The spinal cord 100 is divided into three columns: the dorsal column 102, the ventral column 104, and the lateral column 106. Similarly, the butterfly-shaped gray matter of the spinal cord 100 is divided into the dorsal horn 108, the ventral horn 110, and the lateral horn 112. A ventral median fissure 109 divides the spinal cord 100 into two lateral halves.

A group of motor nerve rootlets (ventral root (VR) nerve fibers) 114 branch off of the ventral horn 110 and combine to form the ventral root 116. Similarly, a group of sensory nerve rootlets (dorsal root (DR) nerve fibers) 118 branch off of the dorsal horn 108 and combine to form the dorsal root 120. The dorsal root 120 and the ventral root 116 combine to form the spinal nerve 122, which innervates peripheral regions (e.g., arms, legs, etc.) of the patient's body. A number of spinal nerves branch off the spinal cord. In each patient, there are eight cervical spinal nerves designated C1-C8, twelve thoracic spinal nerves designated T1-T12, five lumbar spinal nerves designated L1-L5, and five sacral spinal nerves designated S1-S5.

The spinothalamic tract 124 is a neural pathway lying in the ventral portion of the spinal cord. This sensory pathway, which includes bilaterally symmetrical portions lying on either side of the anterior median fissure 126, originates in the spinal cord and carries information to the thalamus concerning pain, temperature, itch, and crude touch. As a primary pathway for pain transmission, the spinothalamic tract 124 offers an attractive target for blocking pain signals. The spinal cord 100 is enclosed by a dura mater 126, with an epidural space 128 surrounding the dura mater 126. The epidural space 128 may be topologically divided into four quadrants: a left ventral-lateral quadrant 130 a, a right ventral-lateral quadrant 130 b, a left dorsal-lateral quadrant 130 c, and a right dorsal-lateral quadrant 130 d.

In conventional SCS techniques, stimulation is applied to the dorsal column 102 of the spinal cord 100 to treat chronic neuropathic pain, and accordingly, stimulation leads are positioned in the dorsal region of the epidural space 128. As opposed to the conventional systems, the embodiments of the present invention modulate the spinothalamic tract 124 of the spinal cord 100 for treating disorders such as chronic neuropathic pain. To this end, the stimulation lead(s) 12 are preferably implanted within the patient lateral to the midline of the spinal cord 100 with the distal ends of the stimulation lead(s) 12 encircling the spinal cord 100 within the epidural space 128, such that the tip(s) of the stimulation lead(s) 12, and thus some of the electrodes 26, are located in either of the ventral-lateral quadrants 130 a, 130 b of the epidural space 128 adjacent the spinothalamic tract 124 of the spinal cord 100.

Thus, electrical stimulation energy can be epidurally applied by the stimulation lead(s) to stimulate the spinothalamic tract 124, thereby treating chronic neuropathic pain. The distal end(s) of the stimulation lead(s) 12 are preferably located at a position along the spinal cord 100 rostral to the dorsal root 120 that innervates the pain region of the patient. In this manner, electrical stimulation occurs at the region of the spinothalamic tract 124 that is rostral to this innervating dorsal root 120, such that the pain signals traveling from this dorsal root 120 are blocked from being conveyed to the brain via the spinothalamic tract 124.

During application of the stimulation energy to the spinothalamic tract 124, the activation threshold of neural structures (e.g., the VR nerve fibers 114 and/or DR nerve fibers 118) relative to the activation threshold of the spinothalamic tract 124 is preferably increased, so that the neural structures are not inadvertently stimulated. As will be described in further detail below, different techniques can be used to increase the activation thresholds of these neural structures relative to the activation threshold of the spinothalamic tract 124. In one embodiment, the stimulation leads(s) are located within the cervical region of the spinal cord, wherein the sensitive VR nerve fibers 114 and DR nerve fibers 118 extend straight out from the spinal cord 100, thereby providing more spatial isolation between these nerve fibers and the spinothalamic tract 124. As such, the distal tip(s) of the stimulation lead(s) 12 may be more easily located between the VR nerve fiber grouping 114 and/or DR nerve fiber grouping 116. The electrodes 26 may be arranged relative to the VR nerve fiber grouping 114 and/or DR nerve fiber grouping 116 in any one of a variety of manners.

For example, in one embodiment illustrated in FIG. 7, a configuration of three electrodes (labeled E_(C) for the center electrode, and E_(F) for the flanking electrodes) is aligned along the longitudinal axis of the spinal cord 100 (i.e., in the rostro-caudal direction) between a pair of VR nerve groupings 114, such that the flanking electrodes E_(F) of the tripole configuration are respectively closest to the VR nerve fiber groupings 114. As seen more clearly in FIG. 7, the distal-most electrodes 26 are placed in the vicinity of, but not quite reaching, the ventral median fissure 109 at the center of the ventral side of the spinal cord 100. Alternatively, the electrode configuration may be obliquely aligned relative to the longitudinal axis of the spinal cord 100 between the ventral roots 116.

In another embodiment illustrated in FIG. 8, a configuration of three electrodes (labeled E_(C) for the center electrode, and E_(F) for the flanking electrodes) is aligned perpendicularly to the longitudinal axis of the spinal cord 100 (i.e., in the medial-lateral direction) between a VR nerve fiber grouping 114 and a DR nerve fiber grouping 118 at the same spinal level on one lateral side of the ventral medial fissure 109, such that the flanking electrodes E_(F) are respectively closest to the VR nerve fiber grouping 114 and the DR nerve fiber grouping 118. Optionally, e.g., in the case of bilateral pain, another configuration of three electrodes can be aligned perpendicularly to the longitudinal axis of the spinal cord 100 between a VR nerve fiber grouping 114 and a DR nerve fiber grouping 118 at the same spinal level on the other lateral side of the ventral medial fissure 109.

The electrode configurations illustrated in FIGS. 7 and 8 can be formed, e.g., using the distal-most electrodes 26 on three side-by-side percutaneous leads 12 (shown in FIG. 3) or the distal-most row of electrodes 26 on the surgical paddle lead 12 (shown in FIG. 4).

As briefly discussed above, the electrodes 26 may be configured in a manner that increases the activation threshold of the VR nerve fiber groupings 114 and/or DR nerve fiber groupings 118 relative to the activation threshold of the spinothalamic tract 124 in order to inhibit the stimulation of VR nerve fiber groupings 114 and/or DR nerve fiber groupings 118 when the spinothalamic tract 124 is stimulated.

The techniques described below apply electrical suppression energy to the nerve fiber groupings to decrease their activation threshold. Preferably, the electrodes 26 are placed as closely as possible to the spinal cord 100 in order to maximize the resolution of the energy transmitted by the electrodes 26; that is, to focus the stimulating effect of the stimulation energy on a target site in the spinothalamic tract 124, and to focus the activation threshold increasing effect of the suppression energy on the nerve fiber groupings 114, 118. Preferably, the proximity of the electrodes 26 to the spinal cord 100 should be less than one-half of the distance between adjacent electrodes 26 to ensure proper resolution. Because the electrodes 26 are typically separated from the spinal cord by the dura 126 and spinal cord fluid, there will typically be some distance between the electrodes 26 and the spinal cord 100, with the distance varying from patient to patient. If implanted within the cervical region of the spine, the proximity of the electrodes 26 to the spinal cord 100 will typically be quite small, and therefore, high resolution can be achieved. If implanted within the thoracic region of the spine, the proximity of the electrodes 26 to the spinal cord 100 will be greater, and therefore, a lower resolution will be achieved.

In one technique for preventing inadvertent stimulation of the nerve fiber groupings 114, 118, the electrodes are configured as a tripole, such that the flanking electrodes E_(F) adjacent the two nerve fiber groupings 114, 118 are anodes, and the center electrode E_(C) is a cathode. In this configuration, electrical stimulation energy conveyed between the cathode(s) and anode(s) creates an electrical field that stimulates the spinothalamic tract 124, while preventing stimulation of the nerve fiber groupings 114, 118. That is, the electrical current sourced by the outer flanking electrodes E_(F) hyperpolarizes the nerve fiber groupings 114, 118, thereby decreasing their activation threshold, while the electrical current sunk by the center electrode E_(C) depolarizes the nerve fibers in the spinothalamic tract 124, thereby creating a locus of stimulation that is confined to the region of the spinothalamic tract 124 adjacent the center electrode E_(C). In effect, the center electrode E_(C) generates the electrical stimulation energy at the spinothalamic tract 124, while the flanking electrodes E_(F) “push” the electrical stimulation energy away from the nerve fiber groupings 114, 118.

It is desirable that the locus of stimulation be as narrow as possible without increasing its depth, thereby stimulating target nerve fibers within the spinothalamic tract, while preventing stimulation of the nerve fiber groupings 114, 118. This would require an increase in the electrical current sourced at the flanking electrodes E_(F). However, this necessarily may result in an increase in the current sunk by the center electrode E_(C), thereby increasing the depth of the locus of stimulation, which may lead to undesirable outcomes (e.g., discomfort or undesirable reflexive activity). As such, a portion of the electrical current sourced at the flanking electrodes E_(F) can be additionally sunk at a remote electrode (e.g., the case or even a therapeutic electrode remote from the tripole configuration), thereby creating a local current imbalance at the target site of the spinothalamic tract 124.

Further details discussing the use of tripolar electrode arrangements to render tissue less excitable to subsequent stimulation are disclosed in U.S. patent application Ser. No. 11/300,963, entitled “Apparatus and Methods for Stimulating Tissue,” which is expressly incorporated herein by reference.

In another technique for preventing inadvertent stimulation of the nerve fiber groupings 114, 118, a sub-threshold, hyperpolarizing conditioning pre-pulse can be applied by flanking electrodes E_(F) to the adjacent nerve fiber groupings 114, 118, and a depolarizing stimulation pulse can be subsequently applied by the center electrode E_(C) to the spinothalamic tract 124. The duration between the conditioning pulse and the stimulation pulse is preferably zero, but at the least should be less than 100 μs, and more preferably, less than 30 μs. The conditioning pre-pulse preferably has a relatively short duration, preferably less than 200 μs, more preferably less than 150 μs, and most preferably less than 75 μs, such that m-gates of the sodium ion channels in the neural axons are closed to render the tissue less excitable to subsequent stimulation. The stimulation pulse preferably has a relatively short duration, e.g., less than 200 μs. Optionally, a relatively long depolarizing conditioning pulse preceding the relatively short hyperpolarizing conditioning pulse can be applied by the flanking electrodes E_(F), such that h-gates of the sodium ion channels in the neural axons are closed to render the tissue even less excitable to subsequent stimulation.

Further details discussing the use of conditioning pre-pulses to render tissue less excitable to subsequent stimulation are disclosed in U.S. patent application Ser. No. 11/752,895, entitled “Short Duration Pre-Pulsing to Reduce Stimulation-Evoked Side-Effects,” which is expressly incorporated herein by reference.

In still another technique for preventing inadvertent stimulation of the nerve fiber groupings 114, 118, electrical background energy is conveyed from the flanking electrodes E_(F) in accordance with at least one stochastic parameter. If the electrical background energy is in the form of pulses, the stochastic parameter may comprise at least one of an interpulse interval, pulse amplitude, pulse shape, and pulse duration. If the electrical background energy is white noise, the stochastic parameter may be a frequency.

The three electrodes can be configured as stimulating electrodes in a tripolar arrangement (center electrode E_(C) as the cathode, and flanking electrodes E_(F) as the anodes) in a manner described above, while the flanking electrodes E_(F) can be configured as background electrodes in a monopolar arrangement with the IPG case 40. Thus, electrical stimulation energy is conveyed between the tripolar arrangement of the electrodes, thereby therapeutically stimulating spinothalamic tract, and electrical background energy is conveyed between the flanking electrodes E_(F) and the IPG case 40, thereby decreasing the excitability of the nerve fiber groupings 114, 118.

Notably, due to the monopolar arrangement, the field strength of the electrical background energy conveyed from the flanking electrodes E_(F) decays at a relatively low rate with distance. As such, the flanking electrodes E_(F) may be relatively far from the nerve fiber groupings 114, 118 for them to modulate the excitability of the nerve fiber groupings 114, 118. Alternatively, the electrical background energy may be conveyed from flanking electrodes E_(F) in bipolar arrangement. However, in this case, due to current shunting, the flanking electrodes E_(F) must be relatively close to the nerve fiber groupings 114, 118. In either case, as a general rule, the excitability of the nerve fiber groupings 114, 118 will be decreased if the magnitude of the electrical background energy is relatively high. Alternatively, electrical background energy with a relatively low magnitude may be conveyed from the center electrode E_(C), thereby increasing the excitability of the spinothalamic tract 124. In effect, the activation threshold of the nerve fiber groupings 114, 118 will be increased relative to the activation threshold of the spinothalamic tract 124 by decreasing the activation threshold of the spinothalamic tract 124.

Because the excitability of the nerve fiber groupings 114, 118 will be decreased by the electrical background energy, the effect that the inadvertent conveyance of the electrical stimulation energy to the nerve fiber groupings 114, 118 will be decreased. In one method, the electrical background energy is set equal to or higher than 10% of the perception threshold of the patient. Preferably, the electrical background energy is sub-threshold (i.e., does not stimulate nerve fibers), but may be supra-threshold if it provides therapy to the patient or can otherwise be tolerated by the patient. Ultimately, the magnitudes of the electrical stimulation energy and electrical background energy can be set relative to each other through trial and error.

Further details discussing the use of electrical background energy to render tissue less excitable to subsequent stimulation are disclosed in U.S. patent application Ser. No. 12/501,127, entitled “System and Method for Reducing Excitability of Dorsal Root Fiber by Introducing Stochastic Background Noise,” which is expressly incorporated herein by reference.

In yet another technique for preventing inadvertent stimulation of the nerve fiber groupings 114, 118, which works particularly well if the nerve fiber groupings 114, 118 extend over the targeted stimulation site of the spinothalamic tract 124 (e.g., in the thoracic region), and are therefore closer to the stimulating electrode(s) than the target site of the spinothalamic tract 124, the electrical stimulation energy takes the form of high frequency (e.g., in the range of 2-30 KHz) time-varying electrical energy (e.g., a sinuosoidal energy), the frequency and amplitude of which may be selected to increase the activation threshold of the relatively close neural structure (in this case, the nerve fiber groupings 114, 118), while stimulating the relatively far neural structure (in this case, the spinothalamic tract 124).

This ability is due, in large part, because the blocking threshold of a nerve fiber by high frequency electrical energy is higher than the threshold at which the nerve fiber is activated by the same high frequency electrical energy. Thus, if the high frequency electrical energy has an amplitude that is higher than the blocking threshold of relatively close nerve fibers, the relatively far nerve fibers at a particular depth will still have a blocking threshold above the amplitude of the high frequency electrical energy, but a stimulation threshold below the amplitude of the high frequency electrical energy. This means that relatively close nerve fibers will be blocked at a stimulation current that activates relatively far nerve fibers.

The desired depth range of blocked nerve fibers and the desired depth range of the stimulated nerve fibers may be tuned by selecting the amplitude and frequency of the high frequency electrical energy. In general, as the amplitude of the high frequency electrical energy is increased, the range of distance in which neural structures are blocked increases, and the range of distance in which the neural structures are stimulated decreases.

Further details discussing the use of high frequency electrical energy to block action potentials in closer neural tissue while stimulating further neural tissue are disclosed in U.S. patent application Ser. No. 12/819,107, entitled “Spatially Selective Nerve Stimulation in High-Frequency Nerve Conduction Block and Recruitment,” which is expressly incorporated herein by reference.

In an optional embodiment, a set of sensors, such as sense amplifiers, accelerometers, chemical sensors, or thermal sensors, may be employed to ensure that the stimulation applied is efficacious, as well as to identify side effects arising from stimulation activity. For example, a sense amplifier coupled to the electrodes 26 may detect ventral or dorsal root activity and subsequently, the stimulation output may be altered to reduce dorsal or ventral root activity.

It should be understood that the SCS system 10 may be utilized alone or in conjunction with other known implantable devices, such as vagal stimulators, pacemakers, defibrillators, sympathetic chain stimulators, brains stimulators, and so on. The SCS system 10 may interact with other stimulator systems, and in one embodiment, all the stimulator systems may be contained within a single system. In such embodiments, the stimulation source and mechanism to control and/or modify stimulation may be applied through a common source. For example, the SCS system 10 may have multiple leads enabling multiple purposes, e.g., an additional stimulation lead or leads positioned over the dorsal column to provide conventional stimulation.

Although particular embodiments of the present inventions have been shown and described, it will be understood that it is not intended to limit the present inventions to the preferred embodiments, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present inventions. Thus, the present inventions are intended to cover alternatives, modifications, and equivalents, which may be included within the spirit and scope of the present inventions as defined by the claims. 

What is claimed is:
 1. A method for treating a patient suffering from chronic neuropathic pain, the method comprising: epidurally applying electrical stimulation energy to a spinothalamic tract of the patient, thereby treating the chronic neuropathic pain.
 2. The method of claim 1, wherein the electrical stimulation energy is applied to a region of the spinothalamic tract of the patient rostral to a spinal nerve that innervates a painful region of the patient.
 3. The method of claim 1, wherein the electrical stimulation energy is applied to the spinothalamic tract by at least one electrode implanted within the ventral-lateral quadrant of an epidural space of the patient.
 4. The method of claim 1, further comprising increasing the activation threshold of a side-effect exhibiting neural structure relative to the activation threshold of the spinothalamic tract of the patient, wherein the electrical stimulation energy is applied to the spinothalamic tract of the patient while the activation threshold of the neural structure relative to the spinothalamic tract is increased, thereby treating the chronic neuropathic pain without stimulating the neural structure.
 5. The method of claim 4, wherein the neural structure is one of a dorsal root (DR) nerve fiber and a ventral root (VR) nerve fiber.
 6. The method of claim 4, wherein neural structure is a cervical spinal nerve.
 7. The method of claim 4, wherein the activation threshold of the neural structure relative to the activation threshold of the spinothalamic tract is increased by applying electrical suppression energy to the neural structure.
 8. The method of claim 7, wherein the electrical stimulation energy is applied to spinothalamic tract by sinking electrical current at a first electrode adjacent the spinothalamic tract, thereby treating the chronic neuropathic pain, and the electrical suppression energy is applied to the neural structure by sourcing at least a portion of the electrical current from a second electrode adjacent the neural structure, thereby increasing the activation threshold of the neural structure.
 9. The method of claim 8, wherein electrical suppression energy is applied to a another neural structure adjacent the spinothalamic tract to increase the activation threshold of the other neural structure by sourcing at least another portion of the electrical current from a third electrode adjacent the other neural structure, wherein the electrical stimulation energy is applied to the spinothalamic tract of the patient while the activation threshold of the other neural structure is increased, thereby treating the chronic neuropathic pain without stimulating the other neural structure.
 10. The method of claim 9, wherein the neural structure is a dorsal root (DR) nerve fiber and the other neural structure is a ventral root (VR) nerve fiber.
 11. The method of claim 9, wherein neural structure is a first ventral root (VR) nerve fiber, and the other neural structure is a second ventral root (VR) nerve fiber.
 12. The method of claim 9, wherein the first electrode is a center electrode, and the second and third electrodes are flanking electrodes of a tripole configuration located between the neural structures.
 13. The method of claim 9, further comprising sinking at least a portion of the electrical current sourced at the second electrode adjacent the neural structure into a remote electrode.
 14. The method of claim 4, wherein the activation threshold of the neural structure is increased by applying a sub-threshold, hyperpolarizing conditioning pre-pulse to the neural structure, and electrical stimulation energy is applied to the spinothalamic tract by applying a depolarizing stimulation pulse to the spinothalamic tract.
 15. The method of claim 14, wherein the conditioning pre-pulse has a duration less than 200 μs.
 16. The method of claim 4, wherein the activation threshold of the neural structure is increased by applying electrical background energy to the neural structure in accordance with at least one stochastic parameter to the neural structure.
 17. The method of claim 16, wherein the electrical background energy comprises pulses, and the at least one stochastic parameter comprises at least one of an interpulse interval, pulse amplitude, pulse shape, and pulse duration.
 18. The method of claim 16, wherein the electrical background energy comprises white noise, and the at least one stochastic parameter comprises a frequency.
 19. The method of claim 4, wherein the activation threshold of the neural structure is increased by applying time-varying electrical energy having a frequency and amplitude that increases the activation threshold of the neural structure.
 20. The method of claim 19, wherein the electrical stimulation energy epidurally applied to the spinothalamic tract is the time-varying electrical energy, and the time-varying electrical energy having a frequency and amplitude that increases the activation threshold of the neural structure, while stimulating the relatively spinothalamic tract. 